Tire burst prediction device

ABSTRACT

A tire burst predicting device which can predict occurrence of bursting of a tire and gives an alarm properly is provided.  
     Break point frequency estimating means  44  estimates a break point frequency on the basis of time series data of wheel speed detected at a wheel speed sensor  32  from frequency response of a first order model to which a transmission characteristic from a road surface disturbance to a wheel speed is approximated. Braking force gradient estimating means estimates a braking force gradient corresponding to the estimated break point frequency, on the basis of a map, which is prestored, representing a relationship between break point frequencies and braking force gradients. Extra high frequency component is eliminated from the estimated braking force gradient by a low pass filter  48  to reduce estimation dispersion. Differentiators  50  detect changing speed of the braking force gradient. Judging means gives an alarm of that there is a possibility of occurrence of bursting of tire in a case in which the detected changing speed is equal to or more than a predetermined threshold value.

TECHNICAL FIELD

[0001] The present invention relates to a tire burst predicting device,and particularly relates to a tire burst predicting device which detectsa ground contact state of a tire and predicts occurrence of bursting ofthe tire.

PRIOR TECHNOLOGY

[0002] What is called a burst, by which a tire is cut, may occur due todeflection being generated on the tire (standing wave phenomenon) and atemperature of the tire increasing when a vehicle travels at high speedin a state in which an air pressure of the tire is low.

[0003] In order to prevent bursting of the tire, technologies have beenproposed in which an air pressure of a tire during traveling is detectedby an air pressure sensor or an air pressure of a tire is estimated fromdetected wheel speed of the tire, and an alarm is given in a case inwhich the detected or estimated air pressure has been lowered.

[0004] For example, in Japanese Patent Application Laid-Open (JP-A) No.7-149123, a device is disclosed, in which a wheel speed of a tire isdetected by a wheel speed sensor, lowering of an air pressure of thetire is judged from the detected wheel speed, and an alarm is given in acase in which it has been judged several times that air pressure hasbeen lowered.

PROBLEMS TO BE SOLVED BY THE INVENTION

[0005] However, there are cases in which proper air pressures forrespective tires are different. Accordingly, an alarm may be incorrectlygiven in a case in which proper air pressure is changed due to tirechanging or tire wear. Further, there is a problem in that a timing ofgiving an alarm may be late or early depending on states of a road and atire or running conditions.

[0006] Further, there is a problem in that, there are cases in which itis impossible to exactly predict a burst and give an alarm by a tire airpressure alarming alone, because there are cases in which bursting of atire occurs when a vehicle runs under conditions outside of an allowablerange, such as when, the vehicle runs at high speed on a road surface ofextremely high temperature, even if the air pressure is proper.

DISCLOSURE OF THE INVENTION

[0007] The present invention has been achieved to solve theabove-described problems, and has an object thereof to provide a tireburst predicting device which can predict occurrence of bursting of atire and gives an alarm with accuracy.

[0008] In order to achieve the object mentioned above, the invention ofclaim 1 is a tire burst predicting device comprising wheel speeddetecting means which detects a speed of a wheel, ground contact stateestimating means which estimates a physical quantity representing aground contact state between the wheel and a road surface on the basisof the detected wheel speed, rate of change detecting means whichdetects a rate of change of the estimated physical quantity representingthe ground contact state between the wheel and the road surface, andpredicting means which predicts occurrence of bursting of the wheel byjudging whether or not the detected rate of change is a value outside ofa predetermined range.

[0009] According to the invention, the wheel speed detecting meansdetects a wheel speed of a vehicle (for example, a four-wheel vehicle).For example, wheel speed sensors, which generate pulses (wheel speedpulses) of a predetermined number for every one rotation of a wheel, areprovided at respective wheels, and the wheel speed can be detected froma counted value or a measured value of a pulse width, in unit time, of awheel speed pulse outputted from the wheel speed sensor.

[0010] The ground contact state estimating means estimates a physicalquantity representing a ground contact state between a wheel and a roadsurface, such as, a physical quantity representing a ground contact areabetween the wheel (the tire) and the road surface, or a friction state(slipperiness) between the wheel and the road surface, on the basis ofthe detected wheel speed. The physical quantity representing thefriction state between the wheel and the road surface is, for example, abraking force gradient. The ground contact state of the wheel canindirectly be ascertained by estimating the braking force gradient. Thebraking force gradient can be estimated from an estimated break pointfrequency. The break point frequency is estimated on the basis of timeseries data of the wheel speed from a frequency response of a firstorder lag model in which transmission characteristics from road surfacedisturbance to wheel speed are approximated, for example. Further,besides the braking force gradient, a driving force gradient when adriving force is applied to the wheel and a road surface μ-gradientwhich represents a gripping state of wheel are both physical quantitiesrepresenting slipperiness between the wheel and the road surface. Theseare physical quantities equivalent to the braking force gradient.Therefore, the driving force gradient or the road surface μ-gradient canbe used instead of the braking force gradient.

[0011] The rate of change detecting means detects a rate of change ofthe estimated physical quantity representing the ground contact statebetween the wheel and the road surface. That is, the rate of changedetecting means detects a quantity of change of the physical quantity,in unit time, representing the ground contact state. Change of a stateof the wheel can be ascertained by detecting the rate of change in thisway. That is, in a case in which the braking force gradient is estimatedas the physical quantity representing the ground contact state, it canbe judged that there is a possibility that the tire will burst when thephysical quantity representing the ground contact state changes rapidlydue to the ground contact area of the wheel increasing as a result of achange in temperature of the wheel or the like, and the estimatedbraking force gradient rapidly increasing or decreasing.

[0012] Then, the predicting means predicts occurrence of bursting of thewheel by judging whether or not the detected rate of change is a valueoutside of a predetermined range. By this, it can be judged that thereis a possibility of occurrence of bursting when the detected rate ofchange is a value outside of the predetermined range.

[0013] Note that it can be judged that there is a possibility ofoccurrence of bursting when a state in which the detected rate of changeis a value outside of the predetermined range continues for apredetermined period or longer.

[0014] In this way, because it is judged whether or not there is apossibility that the wheel will burst from the rate of change of thephysical quantity representing the ground contact state, a degree ofdanger of a burst can be detected with accuracy even in a case in whichwheels having different characteristics are mounted or a road surface ordriving conditions are extremely changed. Further, bursting can bepredicted properly even in a state in which there is a possibility ofoccurrence of bursting of the tire even though an air pressure isappropriate, that is, for example, a state in which the vehicle runs athigh speed on a road surface of extremely high temperature.

[0015] Moreover, there is a problem in that tread peeling or the likeoccurs due to tread heating depending on the tire. However, in this caseas well, the rate of change of the physical quantity representing theground contact state becomes a value outside of the predetermined range,and it is therefore possible to prevent an accident by predictingdanger.

[0016] The alarming means can give an alarm by, for example, sounding analarm or displaying on displaying means, in a case in which the detectedrate of change is a value outside of the predetermined range. As aresult, a driver of the vehicle can easily be made to recognize thatthere is a possibility that the wheel will burst, and can be prompted tosuppress the speed of the vehicle.

[0017] Instead of the alarming, or as well as the alarming, it ispossible that driving force suppressing means suppresses a driving forceof the wheel in a case in which the detected rate of change is a valueoutside of the predetermined range. As a result, the speed of thevehicle can be suppressed and bursting of the wheel can be prevented.

[0018] Further, the ground contact state estimating means can bestructured by smoothing means which smoothes the physical quantityrepresenting the ground contact state, and a differentiator whichdifferentiates the smoothed physical quantity representing the groundcontact state. For example, a low pass filter can be used as thesmoothing means. In this way, by smoothing and differentiating thephysical quantity representing the ground contact state, extra highfrequency components are eliminated, and it is possible to detect onlychange in the physical quantity representing the ground contact stategenerated, for example, due to change in wheel temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a block diagram showing a dynamic model of a wheelresonance system of embodiments of the present invention.

[0020]FIG. 2 is a diagram showing a relationship between a slip speedand a road surface μ.

[0021]FIG. 3 is a gain diagram showing frequency responses from roadsurface disturbances to wheel speed.

[0022]FIG. 4 is a schematic block diagram of a tire bust predictingdevice of the embodiment of the present invention.

[0023]FIG. 5 is a flow chart showing algorithm for estimating a breakpoint frequency in the embodiment of the present invention.

[0024]FIG. 6 is a gain diagram showing frequency responses from roadsurface disturbances to wheel speed in a first-order lag model.

[0025]FIG. 7 is a diagram showing a relationship between break pointfrequency and braking force gradient.

[0026]FIG. 8 is a flow chart of control routine executed by judgingmeans.

[0027]FIG. 9 is a schematic block diagram of a tire bust predictingdevice of another example.

[0028]FIG. 10 is a schematic block diagram of wheel speed frequencycharacteristic quantity estimating means.

BEST MODES FOR IMPLEMENTING THE INVENTION

[0029] [A First Embodiment]

[0030] A first embodiment of the present invention will be describedhereinafter. First, a principle, in which a gradient of braking force,as a physical quantity representing a ground contact state of a tire, isestimated, will be described.

[0031] As shown in FIG. 1, a dynamic model of a wheel resonance systemcan be represented by a model in which torsional spring elements 14 and16 of a tire, having respective spring constants K1 and K2, areinterposed between a rim 10 and a belt 12 and in which a suspension,provided by connecting a spring element 18 having a spring constant K3in parallel with a damper 20, is interposed between the rim 10 and avehicle body. In this model, a disturbance from the road surface (roadsurface disturbance) is transmitted from the belt 12 through the springelements 14 and 16 to the rim 10, to affect a wheel speed ω, and istransmitted to the vehicle body through the suspension element.

[0032] A description is now given of a relationship between the brakingforce gradient and a wheel speed frequency characteristic quantityrepresenting a following frequency of characteristics of transmissionfrom the road surface disturbance to the wheel speed, using a fifthorder full wheel model, in which a first order wheel deceleratingmotion, a second order longitudinal direction suspension resonance, anda second order tire rotation resonance are integrated. As shown in FIG.2, the braking force gradient is represented by a gradient (a slope) ofa tangent of a curve representing a relationship between a slip speed(or a slip rate) and the braking force.

[0033]FIG. 3 is a gain diagram showing frequency responses from roadsurface disturbance to the wheel speed for ranges from a limit brakingrange to a low slip range where there is some margin for tirecharacteristics, namely, for ranges from a range at which the brakingforce gradient is 300 Ns/m to a range at which the braking forcegradient is 10000 Ns/m. That is, the diagram shows the relationshipbetween frequency and gain of amplitude of the wheel speed with respectto amplitude of the road surface disturbance.

[0034] In FIG. 3, the frequency characteristics of the wheel speedindicate that, in a case in which the braking force gradient isrelatively small, such as near the limit of friction between the tireand the road, the gain is large in a low frequency range and the gain issmall in a high frequency range. Namely, for the range where the brakingforce gradient is relatively small, there is a big difference betweenthe gain in the low frequency range and the gain in the high frequencyrange, the difference representing the wheel speed frequencycharacteristic quantity.

[0035] In contrast, the gain in the low frequency range for the rangewhere the braking force gradient is relatively large, such as at a timeof stationary traveling, is much smaller compared to the gain in the lowfrequency range for the range where the braking force gradient isrelatively small, in the wheel speed frequency characteristics. On theother hand, in the high frequency range, the gain for the range wherethe braking force gradient is relatively large is not much smaller thanthe gain for the range where the braking force gradient is relativelysmall because of the influence of generation of rotational resonance ofthe tire (near 40 Hz) and the like. As the results, for the range wherethe braking force gradient is relatively large, the wheel speedfrequency characteristic quantity is small. In the similar way, a wheelspeed frequency characteristic quantity representing a differencebetween a vibration level of a wheel speed signal in the low frequencyrange and a vibration level of a wheel speed signal in the highfrequency range changes similarly to the wheel speed frequencycharacteristic quantity representing difference between the gain in thelow frequency range and the gain in the high frequency range, mentionedabove.

[0036] It is apparent from the above that the wheel speed frequencycharacteristic quantity representing the difference (or a ratio) betweenthe gain in the low frequency range and the gain in the high frequencyrange, or the difference (or a ratio) between the wheel speed signalvibration level in the low frequency range and the wheel speed signalvibration level in the high frequency range decreases as the brakingforce gradient increases. Utilizing this property, the braking forcegradient can be estimated from the wheel frequency speed characteristicquantity.

[0037] Noting to the frequency band near 40 Hz in FIG. 3 at whichrotational resonance of the tire occurs, the greater the braking forcegradient, the sharper the peak resonance waveform of rotationalresonance of the tire. Further, as the braking force gradient becomesgreater, the overall frequency characteristics (the overall waveform) ofthe peak resonance waveform of rotational resonance of the tire moves tohigher frequency side.

[0038] Namely, if the wheel characteristics are approximated by afirst-order lag model, it can be understood that a break point frequencybecomes higher as the braking force gradient becomes larger, as shown inFIG. 6. It is therefore possible to estimate the braking force gradientfrom a value of the wheel speed frequency characteristic quantityrepresenting following frequency of transmission characteristics fromthe road disturbance to the wheel speed, by approximating thecharacteristics of the wheel with a first-order lag model and estimatingthe break point frequency, which is a frequency at which the gainchanges from a value in a predetermined range to a value out of thepredetermined range, as the wheel speed frequency characteristicquantity. Lag models of the second and third orders and the like havecharacteristics substantially similar to those of the first-order lagmodel. Therefore, it is possible to estimate the braking force gradientfrom the value of the wheel speed frequency characteristic quantity, byapproximating wheel characteristics with the lower order lag model andestimating the wheel speed frequency characteristic quantity thereof.

[0039] As well as the braking force gradient in which the braking forceis applied to the tire, a driving force gradient in which the drivingforce is applied to the tire are all physical quantities representingslipperiness between the tire and the road surface, and are physicalquantities which are equivalent to a road surface μ-gradientrepresenting a grip state of the tire. Accordingly, one of the brakingforce gradient, which is the gradient of the tangent of the curverepresenting the relationship between the slip speed or the slip rateand the braking force, the driving force gradient, which is a gradientof a tangent of a curve representing a relationship between a slip speedor a slip rate and a driving force, and the road surface μ-gradient,which is a gradient of a tangent of a curve representing a relationshipbetween a slip speed or a slip rate and a road surface μ, can beestimated as the physical quantity representing slipperiness of roadsurface, from wheel speed frequency characteristic quantity.

[0040] The first embodiment will be described hereinafter, in which thebreak point frequency is estimated as the wheel speed frequencycharacteristic quantity, the braking force gradient is estimated as thephysical quantity representing ground contact state from the estimatedbreak point frequency, and occurrence of bursting of a tire is judged onthe basis of the estimated braking force gradient.

[0041] As shown in FIG. 4, a tire burst predicting device 30 includeswheel speed sensors as wheel speed detecting means 32F1, 32F2, 32R1,32R2, ground contact state detecting means as ground contact stateestimating means 34, ground contact state rate of change detecting meansas rate of change detecting means 36, judging means 38, alarming means40 and driving force suppressing control means 42. The wheel speeddetecting means 32F1, 32F2, 32R1, 32R2 detect respective wheel speeds ofa front-right wheel, a rear-right wheel, a front-left wheel and arear-left wheel of a vehicle not shown in the drawing in a predeterminedsampling period and output time series data of the wheel speeds as wheelspeed signals. The ground contact state detecting means 34 is forindirectly detecting physical quantities representing ground contactstates for respective tires, namely, physical quantities representingadhesion state between the tire and the road surface. The ground contactstate rate of change detecting means 36 detects rate of changes of thedetected physical quantities representing ground contact states forrespective tires. The judging means judges whether or not there is apossibility in which the tire will burst on the basis of the detectedrate of changes of the physical quantities representing ground contactstates for respective tires. The warning means 40 gives a warning, byinstruction from the judging means 38, of that there is a possibility inwhich the tire will burst. The driving force suppressing control means42 controls suppressing, by instruction from the judging means 38,driving force of the tire.

[0042] The wheel speed sensor 32F1 outputs a wheel speed of thefront-left wheel, the wheel speed sensor 32F2 outputs a wheel speed ofthe front-right wheel, the wheel speed sensor 32R1 outputs a wheel speedof the rear-left wheel, the wheel speed sensor 32R2 outputs a wheelspeed of the rear-right wheel. Also, as shown in FIG. 4, break pointfrequency estimating means 44, braking force gradient estimating means46, low-pass filters 48 and differentiators 50 are provided atrespective tires of the front-left wheel, the front-right wheel, therear-left wheel and the rear-right wheel.

[0043] The ground contact state detecting means 34 includes, as anexample, the break point frequency estimating means 44 and the brakingforce gradient estimating means 46. The break point frequency estimatingmeans 44 estimates a break point frequency as a frequency at which, in again diagram that represents a frequency response of a model of which atransmission characteristics from road surface disturbance to wheelspeed is approximated to first order lag model on the basis of timeseries data of the wheel speed, gain changes from a constant value(changes from a value in a predetermined range to a value out of thepredetermined range). The braking force gradient estimating means 46estimates a braking force gradient corresponding to the estimated breakpoint frequency, on the basis of a map, which is prestored, representinga relationship between break point frequencies and the braking forcegradients.

[0044] The ground contact state rate of change detecting means 36includes, as an example, the low pass filters 48 and the differentiators50. The low pass filters 48 are for reducing estimation variations ofthe estimated braking force gradients. The differentiators 50 are fordetecting rate of change of the braking force gradient.

[0045] Next, the estimation of the break point frequency by the breakpoint frequency estimating means 44 and the estimation of the brakingforce gradient by the braking force gradient estimating means 46 will bedescribed.

[0046] In the break point frequency estimating means 44 of the presentembodiment, a break point frequency in a first-order lag model using aleast squares method is identified with an assumption that a whitedisturbance, that is, a disturbance including all frequencies, isinputted to the tire from the road surface.

[0047]FIG. 5 shows an algorithm for identifying the break pointfrequency, and FIG. 6 is gain diagrams of first-order lag modelassociated with break point frequencies identified by the algorithm inFIG. 5 when a white disturbance is applied to the full wheel model inFIG. 1.

[0048] First, the algorithm for identifying the break point frequencywill be described with reference to FIG. 5. At step 100, data, which isthe time series data of the wheel speed detected by the wheel speedsensor 32 with white disturbance added, is acquired and then subjectedto a pre-process at step 102 using a second-order Butterworth filter, bya filter having, for example, a 2 Hz high-pass filter and, for example,a 20 Hz low-pass filter. Steady components of acceleration of the wheelcan be eliminated by inputting the wheel speed signal to the high-passfilter to perform high-pass filtering, and a smoothing process isperformed on the wheel speed signal by the low-pass filtering.

[0049] At subsequent step 104, time series data of a break pointfrequency is estimated from the pre-processed time series data of thewheel speed using the on-line least squares method. First, the timeseries data of the wheel speed which has been detected by the wheelspeed sensor 32, on a discrete basis at sampling period τ, has beensubjected to the pre-process by the filter at step 102. Therefore, thistime series data of the wheel speed is represented by ω[k] (k representssampling times based on the sampling period τ as a unit and takes values1, 2, . . . ). Then, the following steps 1 and 2 are repeated. Thus,time series data of a break point frequency is estimated from thedetected time series data of the wheel speed.

[0050] [Formula 1]

[0051] Step 1:

φ[k]=τ{ω[k−1]−ω[k−2]}  (1)

y[k]=−ω[k]+2ω[k−1]−ω[k−2]  (2)

[0052] φ[k] in Equation (1) is a value obtained by multiplying thequantity of a change in the wheel speed in one sample period by sampleperiod τ (a physical quantity associated with a change in the wheelspeed), and y[k] (a physical quantity associated with a change in achange in the wheel speed, namely, a physical quantity associated with achange in a wheel acceleration) in Equation (2) is a quantity of changein one sample period (ω[k−1]−ω[k−2]−(ω[k]−ω[k−1])) in changes in thewheel speed in one sample period (ω[k−1]−ω[k−2], ω[k]−ω[k−1]).

[0053] [Formula 2]

[0054] Step 2:

θ[k]=θ[k−1]+L[k] (y[k]−φ[k] ^(T) ·θ[k−1])   (3)

[0055] Here, $\begin{matrix}{{L\lbrack k\rbrack} = \frac{{P\left\lbrack {k - 1} \right\rbrack}{\varphi \lbrack k\rbrack}}{\lambda + {{\varphi \lbrack k\rbrack}^{T}{P\left\lbrack {k - 1} \right\rbrack}{\varphi \lbrack k\rbrack}}}} & (4) \\{{P\lbrack k\rbrack} = {\frac{1}{\lambda}\left\lbrack {{P\left\lbrack {k - 1} \right\rbrack} - \frac{{P\left\lbrack {k - 1} \right\rbrack}{\varphi \lbrack k\rbrack}{\varphi \lbrack k\rbrack}^{T}{P\left\lbrack {k - 1} \right\rbrack}}{\lambda + {{\varphi \lbrack k\rbrack}^{T}{P\left\lbrack {k - 1} \right\rbrack}{\varphi \lbrack k\rbrack}}}} \right\rbrack}} & (5)\end{matrix}$

[0056] An estimated value θ, i.e., a break point frequency, is estimatedfrom the above recurrence formulae. λ in Equations (4) and (5)represents a forgetting coefficient which indicates the degree ofelimination of previous data (for example, λ=0.98), and T representstransposition of a matrix.

[0057] θ[k] in Equation (3) is a physical quantity representing historyof physical quantity associated with the change in the wheel speed andhistory of physical quantity associated with the change in the change inthe wheel speed, i.e., the change in the wheel acceleration.

[0058] While an example of estimation of the break point frequency usingthe on-line least squares method has been described above, the breakpoint frequency can be estimated using other on-line methods, such asthe instrumental variable method and the like.

[0059]FIG. 6 shows example of result of estimation of the break pointfrequency in first-order lag model, estimated as described above. Aswill be understood from the gain diagram in FIG. 6, each gain of anapproximated first-order lag model is identified as a characteristicsthat passes through a gain at an antiresonant point (near 40 Hz) andsteady gain in a full wheel model gain diagram for each braking forcegradient other than 300 Ns/m. Suspension longitudinal directionresonance near 15 Hz and resonance characteristics of rotationalvibration of the tire near 40 Hz are ignored as a result of use of thelower order model. When a braking force gradient is small such as 300Ns/m, no resonance is observed because no antiresonant point is passedin the first-order lag model, which indicates that the vibrationcharacteristics of the first-order lag model and the characteristics ofthe full wheel model agree with each other well. The reason for this isthat a wheel deceleration motion model is dominant in a braking regionnear the limit, where the braking force gradient is 300 Ns/m or less,because there is less influence of suspension longitudinal directionresonance or resonance of rotational vibration of the tire. It istherefore considered that motion of the wheel can be approximated by thefollowing wheel deceleration motion model in this region near the limit.

[0060] [Formula 3]

{umlaut over (v)} _(w) =−kR _(C) ² /J {dot over (v)} _(w) +w   (6)

[0061] where v_(w) represents a wheel speed (m/s); w represents a roadsurface disturbance; k represents a braking force gradient (Ns/m); R_(C)represents an effective radius of the tire (m); J represents moment ofinertia of a vehicle; and a coefficient of v_(w) represents the breakpoint frequency.

[0062] Equation (6) indicates that the following relationship existsbetween a break point frequency ω₀ and a braking force gradient in thelimit region.

[0063] [Formula 4] $\begin{matrix}{\omega_{0} = \frac{k\quad R_{c}^{2}}{J}} & (7)\end{matrix}$

[0064] In a low slip region, a relationship shown in FIG. 7 can bederived by using a least squares method. FIG. 7 shows the relationshipbetween braking force gradients in the full wheel model and break pointfrequencies identified from wheel speed data with white disturbancebeing added. The unit of the break point frequencies in FIG. 7 isrepresented [rad/s]. The braking force gradient monotonously increasesas the break point frequency increases. It is therefore possible toestimate the braking force gradient by estimating (identification)result of the break point frequency by that the relationship betweenbraking force gradients and break point frequencies shown in FIG. 7 isstored as a map in a memory of the braking force gradient estimatingmeans 46, and the braking force gradient corresponding the break pointfrequency estimated by the break point frequency estimating means 44 onthe basis of the wheel speed signal by using the map is calculated.

[0065] Extra high frequency component is eliminated from the estimatedbraking force gradient of each tire by the low pass filter 48 to smooththe braking force gradient. The braking force gradient is outputted tothe differentiator 50. In the differentiator 50, the braking forcegradient is differentiated, and a rate of change of the braking forcegradient, that is, a changing quantity of the braking force gradient inan unit time, is outputted to the judging means 38.

[0066] Because the rate of change of the braking force gradient isdetected by that the estimated braking force gradient is smoothed by thelow pass filter 48 and is differentiated by the differentiator 50 inthis way, change in tire characteristic due to change of temperature ofa tire tread, that is, change of the braking force gradient in time onlycan be detected.

[0067] Next, control routine executed by the judging means 38 will beexplained. In step 202 shown in FIG. 8, the judging means 38 inputs therate of change of the braking force gradient of each tire.

[0068] Then, in step 202, it is each compared whether or not theinputted rate of change of the braking force gradient of each tire isequal to or more than a predetermined threshold value. That is, it isjudged whether or not the braking force gradient becomes large or smallrapidly with a speed more than or equal to a predetermined speed. Thethreshold value is set as a value of which, for example, there is apossibility of occurrence of bursting if the rate of change becomeslarger than the value.

[0069] If any one of the rate of changes of the braking force gradientsof respective tires is equal to or more than the predetermined thresholdvalue, the alarming means 40 alarms of that there is a possibility ofoccurrence of bursting of the tire, and urges a driver to stop driving.A case in which the rate of change of the braking force gradient of thetire becomes equal to or more than the predetermined threshold value is,for example, a case in which the braking force gradient becomes rapidlylarger or small due to that the temperature of the tire increases and acontact area between the tire and the road surface becomes large. Inthis case, it can be judged that there is a high possibility that thetire will burst.

[0070] The alarm may be given by, for example, an alarm sound, or bydisplaying a position of the tire of which the rate of change of thebraking force gradient is equal to or more than the predeterminedthreshold value on a display panel. Also, the alarm may be given bycombination of both cases.

[0071] Instead of alarming, or as well as alarming, it is possible thatspeed or driving force of the vehicle is suppressed by the driving forcesuppressing control means 42. By this, burst of tire can be prevented.

[0072] The same predetermined threshold value may be set for each tire.Alternatively, because each durability against burst of the respectivetires are different depending on abrasion states of the tires, differentthreshold values may be set for the respective tires in accordance withrespective abrasion states of the tires. Also, because durabilityagainst burst of tire becomes different in accordance with a type oftire, the threshold value may be set in accordance with mounted tire.

[0073] Also, a difference between the rate of changes of the brakingforce gradients of the right wheel and the left wheel, to be concretely,a difference between an output of the differentiator 50 corresponding tothe wheel speed sensor 32F1 and an output of the differentiator 50corresponding to the wheel speed sensor 32F2 and a difference between anoutput of the differentiator 50 corresponding to the wheel speed sensor32R1 and an output of the differentiator 50 corresponding to the wheelspeed sensor 32R2, can be obtained, and the alarm can be given when thedifference is equal to or more than a predetermined threshold value,that is, when the difference between the rate of change of the brakingforce gradient of the right wheel and the rate of change of the brakingforce gradient of the left wheel is greatly different. Also, in thesimilar way, a difference between the rate of changes of the brakingforce gradients of the front wheel and the rear wheel, to be concretely,a difference between an output of the differentiator 50 corresponding tothe wheel speed sensor 32F1 and an output of the differentiator 50corresponding to the wheel speed sensor 32R1 and a difference between anoutput of the differentiator 50 corresponding to the wheel speed sensor32F2 and an output of the differentiator 50 corresponding to the wheelspeed sensor 32R2, can be obtained, and the alarm can be given when thedifference is equal to or more than a predetermined threshold value,that is, when the difference between the rate of change of the brakingforce gradient of the front wheel and the rate of change of the brakingforce gradient of the rear wheel is greatly different.

[0074] In this way, the alarm can be given appropriately without beingaffected by change of the braking force gradient caused by disturbancessuch as a road surface state or an attitude of vehicle, by judgingwhether or not there is a possibility of occurrence of bursting byobtaining the difference between the rate of changes of the brakingforce gradients of the right wheel and the left wheel and/or thedifference between the rate of changes of the braking force gradients ofthe front wheel and the rear wheel. Further, robust can be improved bysynthetically judging whether or not there is a possibility ofoccurrence of bursting by determining the rate of change of the brakingforce gradient of each tire, the difference between the rate of changesof the braking force gradients of the front wheel and the rear wheel,and the difference between the rate of changes of the braking forcegradients of the right wheel and the left wheel.

[0075] Further, a difference between the braking force gradients of theright wheel and the left wheel or a difference between the braking forcegradients of the front wheel and the rear wheel, instead of thedifference between the rate of changes of the braking force gradients ofthe right wheel and the left wheel or the difference between the rate ofchanges of the braking force gradients of the front wheel and the rearwheel, can be obtained, and a rate of change of the difference can beobtained. Then, it can be judged whether or not the rate of change ofthe difference is equal to or more than a predetermined threshold value,and the alarm can be given when the difference is equal to or more thanthe predetermined threshold value.

[0076] To be concretely, as shown in FIG. 9, for a case of alarming ofthat there is a possibility of occurrence of bursting of one of the leftand the right tires, a subtraction circuit 47 calculates a differencebetween a braking force gradient of the left-front wheel estimated bythe braking force gradient estimating means 46 corresponding to thewheel speed sensor 32F1 and a braking force gradient of the right-frontwheel estimated by the braking force gradient estimating means 46corresponding to the wheel speed sensor 32F2. A rate of change of thedifference is detected by the ground contact state rate of changedetecting means 36. The alarm can be performed by the alarm means 40when the difference outputted from the ground contact state rate ofchange detecting means 36 is equal to or more than the predeterminedthreshold value by the judging means 38. Note that, for a case ofalarming of that there is a possibility of occurrence of bursting of oneof the front and rear tires, the alarm can be given by a structuresimilar to the structure mentioned above.

[0077] In this way, because it is judged whether or not there is apossibility of occurrence of bursting of the tire from the rate ofchange of the braking force gradient of the tire, a critical state ofburst can be properly detected without being affected by offset orchange in sensitivity even when tires having different characteristicsare mounted or a road surface or driving condition is extremely changed.In other words, the critical state of burst can be accurately detectedwithout the need for compensating offset or change in sensitivity inaccordance with difference of the tires, the road surface or drivingcondition.

[0078] Further, the alarm can be properly given even in a state in whichthere is a possibility of occurrence of bursting of the tire even thoughan air pressure is appropriate, that is, for example, a state in whichthe vehicle runs at high speed on the road surface of extremely hightemperature.

[0079] Further, in a case in which a tire blowouts and a tire airpressure alarming device is operated, and the vehicle run for itself toa repair shop, a secondary alarm can be given if there is a highpossibility of occurrence of bursting of the tire.

[0080] It is possible that the rate of change of the braking forcegradient in a normal state is stored and the threshold value is obtainedby learning from the rate of change of the braking force gradients inpast. This learning may be performed by learning a mean value of therate of changes of the braking force gradients for respective tires. Inthis case, it is preferable that there are not large difference amongthe rate of changes of the braking force gradients for respective tires.It can be detected properly at an early stage that there is a highpossibility of occurrence of bursting and wrong alarm or wrong operationcan be prevented by learning threshold value for judging whether or notthere is a possibility of occurrence of bursting in this way.

[0081] Further, in the present embodiment, the braking force gradient isobtained from estimated break point frequency and burst of the tire isjudged from the rate of change of the obtained braking force gradient.However, it is possible that burst of the tire is judged directly fromthe break point frequency. In this case, in a case of the structureshown in FIG. 4, the braking force gradient estimating means 46 areomitted, and the break point frequency estimating means 44 are directlyconnected to the low-pass filters 48. By this, the break point frequencyoutputted from the break point frequency estimating means 44 is smoothedby the low-pass filter 48 and is outputted to the differentiator 50, andrate of change of the break point frequency is outputted from thedifferentiator 50. The judging means 38 judges whether or not the rateof change of the break point frequency outputted from the differentiator50 is more than or equal to a predetermined threshold value, and thealarm is given by the alarm means 40 when the rate of change of thebreak point frequency is more than or equal to the predeterminedthreshold value.

[0082] In a case of the structure shown in FIG. 9, the braking forcegradient estimating means 46 are omitted, and the break point frequencyestimating means 44 are directly connected to the subtraction circuit47. By this, a difference between the break point frequencies of theleft and the right tires is calculated at the subtraction circuit 47,and the difference is outputted to the low-pass filter 48. The judgingmeans 38 judges whether or not the rate of change of the difference ofthe break point frequencies outputted from the differentiator 50 is morethan or equal to a predetermined threshold value, and the alarm is givenby the alarm means 40 when the rate of change of the difference of thebreak point frequencies is more than or equal to the predeterminedthreshold value.

[0083] [A Second Embodiment]

[0084] Next, a second embodiment will be described. In the presentembodiment, a difference between a vibration level in a low frequencyrange and a vibration level in a high frequency range, instead of thebreak point frequency, is used as the wheel speed frequencycharacteristic quantity. μ-gradient of the road surface is estimated andit is judged whether or not there is a possibility of occurrence ofbursting of the tire from the estimated μ-gradient of the road surface.The same reference numerals are applied to the same sections as those ofthe first embodiment and the descriptions thereof are omitted.

[0085] As shown in FIG. 10, wheel speed frequency characteristicquantity estimating means 44 corresponding to the break point frequencyestimating means 44 in the first embodiment is structured by lowfrequency characteristic quantity calculating means including aband-pass filter 60A for extracting a wheel speed signal in a lowfrequency range and first vibration level calculating means 62A forcalculating a vibration level from the wheel speed signal afterfiltering, high frequency characteristic quantity calculating meansincluding a band-pass filter 60B for extracting a wheel speed signal ina high frequency range and second vibration level calculating means 62Bfor calculating a vibration level from the wheel speed signal afterfiltering, and characteristic quantity calculating means 64 foroutputting a difference between a low frequency characteristic quantitycalculated by the low frequency characteristic quantity calculatingmeans and a high frequency characteristic quantity calculated by thehigh frequency characteristic quantity calculating means, to serve asthe wheel speed frequency characteristic quantity. A road surfaceμ-gradient estimating means (not shown in the drawing) corresponding tothe braking force gradient estimating means 46 in the first embodimentis connected to an output of the characteristic quantity calculatingmeans 64.

[0086] The band pass filter 60A of the low frequency characteristicquantity calculating means is set with a transmission frequency so as totransmit wheel speed signals in a region of relatively low frequency ofwheel speed motion. The band pass filter in the present embodiment isset to transmit wheel speed signals at frequencies from 15 to 50 Hz.Further, the band pass filter 60B of the high frequency characteristicquantity calculating means is set with a transmission frequency so as totransmit wheel speed signals in a region of relatively high frequency ofwheel speed motion. The band pass filter 60B in the present embodimentis set to transmit wheel speed signals at frequencies from 30 to 50 Hz.

[0087] The vibration level detection means 62A squares the wheel speedsignal transmitted by the band pass filter 60A and represents indecibels and outputs the signal to serve as the low frequencycharacteristic quantity. The vibration level detection means 62B squaresa wheel speed signal transmitted by the band pass filter 60B andrepresents in decibels and outputs the signal to serve as the highfrequency characteristic quantity.

[0088] The characteristic quantity calculating means 64 outputs thedifference between the low frequency characteristic quantity and thehigh frequency characteristic quantity to serve as the wheel speedfrequency characteristic quantity.

[0089] As previously described with reference to FIG. 3, in a regionwhere the μ-gradient of the road surface (a value equivalent to thebraking force gradient in FIG. 3) is relatively small, such as near thelimit, the frequency characteristics of the wheel speed exhibit highgain in the low frequency range and low gain in the high frequencyrange. Therefore, the wheel speed frequency characteristic quantityindicating the difference between the gain in the low frequency rangeand the gain in the high frequency range is large. In contrast, in aregion where the μ-gradient of the road surface is relatively large,such as at steady travelling, the frequency characteristics of the wheelspeed signals exhibit that the gain in low frequency range is smallerthan that for the region where the μ-gradient of the road surface isrelatively small. In contrast, gain in high frequency range does not somuch smaller compared to that for the region where the road surfaceμ-gradient is relatively small, for reasons such as affection by theoccurrence of rotational resonance of the tire. This leads to a smallwheel speed frequency characteristic quantity. Therefore, the wheelspeed frequency characteristic quantity indicating the differencebetween the vibration level in the low frequency range and the vibrationlevel in the high frequency range decreases as the road surfaceμ-gradient increases. The road surface μ-gradient is estimated from thewheel speed frequency characteristic quantity by utilizing thisproperty.

[0090] In the road surface μ-gradient estimating means of the presentembodiment, a relationship between the road surface μ-gradient and thewheel speed frequency characteristic quantity representing thedifference between the vibration level in the low frequency range andthe vibration level in the high frequency range is stored as a map inadvance, by utilizing the property in which the wheel speed frequencycharacteristic quantity decreases as the road surface μ-gradientincreases, and the road surface μ-gradient is estimated from theestimated wheel speed frequency characteristic quantity and the map.

[0091] In the present embodiment, it has been described that the roadsurface μ-gradient estimating means (not shown in the drawing)corresponding to the braking force gradient estimating means 46 in thefirst embodiment is connected to the output of the characteristicquantity calculating means 64. In a case of the structure shown in FIG.4, the output of the characteristic quantity calculating means 64 isdirectly connected to the low-pass filters 48. In a case of thestructure shown in FIG. 9, the output of the characteristic quantitycalculating means 64 is directly connected to the subtraction circuit47. Namely, it can be directly judged whether or not there is apossibility of occurrence of bursting of the tire from the estimatedμ-gradient of the road surface from the wheel speed frequencycharacteristic quantity representing the difference between thevibration level in the low frequency range and the vibration level inthe high frequency range.

[0092] [Effect of the Invention]

[0093] As described above, the present invention has an effect that theoccurrence of bursting of the tire can be properly predicted.

1. A tire burst predicting device comprising: wheel speed detectingmeans which detects a speed of a wheel; ground contact state estimatingmeans which estimates a physical quantity representing a ground contactstate between the wheel and a road surface on the basis of the detectedwheel speed; rate of change detecting means which detects a rate ofchange of the estimated physical quantity representing the groundcontact state between the wheel and the road surface; and predictingmeans which predicts occurrence of bursting of the wheel by judgingwhether or not the detected rate of change is a value outside of apredetermined range.
 2. The tire burst predicting device of claim 1further comprising alarming means which gives an alarm relating tobursting of the wheel, wherein the predicting means judges whether ornot the detected rate of change is a value outside of the predeterminedrange, and the alarming means gives the alarm in a case in which thedetected rate of change is a value outside of the predetermined range.3. The tire burst predicting device of claim 1 or 2 further comprisingdriving force suppressing means which suppresses a driving force of thewheel, wherein the predicting means judges whether or not the detectedrate of change is a value outside of the predetermined range, and thedriving force suppressing means suppresses the driving force of thewheel in a case in which the detected rate of change is a value outsideof the predetermined range.
 4. The tire burst predicting device of anyone of claims 1, 2 or 3, wherein the ground contact state estimatingmeans comprises smoothing means which smoothes the physical quantityrepresenting the ground contact state, and a differentiator whichdifferentiates the smoothed physical quantity representing the groundcontact state.